Question: $ D = \left[\begin{array}{rrr}2 & 0 & 1 \\ 0 & 1 & 2 \\ 1 & 2 & 2\end{array}\right]$ What is $ D^{-1}$ ?
$ = \left[\begin{array}{rrr}\frac{2}{5} & -\frac{2}{5} & \frac{1}{5} \\ -\frac{2}{5} & -\frac{3}{5} & \frac{4}{5} \\ \frac{1}{5} & \frac{4}{5} & -\frac{2}{5}\end{array}\right]$